1. Field of the Invention
The present invention relates to a control device and a control method for a general-purpose engine, for controlling an rpm (rotation per minute) of a spark-ignition engine including an electronic throttle provided in an intake passage, and more particularly, to an rpm control device and an rpm control method for a general-purpose engine, for performing droop control by reducing an engine rpm along with an increase in engine load.
2. Description of the Related Art
An electronic governor mechanism is conventionally known as a control device for a general-purpose engine. For example, an electronic governor mechanism for a compression-ignition engine (diesel engine) is a control device which controls a fuel injection amount of the engine so as to stabilize an engine rpm at a target rpm. As methods for controlling the fuel injection amount, isochronous control and droop control are known.
In the isochronous control, when a load of the engine fluctuates to generate a deviation between the target rpm and an actual rpm, the fuel injection amount is controlled so as to eliminate the deviation. The compression-ignition engine corrects the fuel injection amount to control the rpm so as to maintain it at the target rpm.
On the other hand, in the droop control, when the load is applied to the engine, the fuel injection amount is controlled so that the engine rpm is reduced in accordance with a magnitude of the load. In the compression-ignition engine, the fuel amount is increased by a predetermined amount in accordance with the deviation between the target rpm and the actual rpm. When an external load overwhelms a torque generated by the engine, the engine rpm is controlled so as to be reduced (see Japanese Patent Application Laid-open No. 2008-231939, for example).
Virtual droop control corresponding to the combination of the isochronous control and the droop control has also been proposed (see Japanese Patent Application Laid-open No. 2000-110635, for example).
In the virtual droop control described in Japanese Patent Application Laid-open No. 2000-110635, control characteristics of a mechanical governor are realized by an electronic governor. In vessel engines, farm-vehicle engines, and construction-work vehicle engines, the engine rpm is reduced in accordance with an increase in engine load so that the degree of application of the load can be obtained based on the reduction in engine rpm. In the virtual droop control, engine-operation performance suited for a feeling of a driver is to be realized in the manner described above. Moreover, although a feeling of discomfort is provided to the driver when sudden acceleration or deceleration is performed at the time of acceleration or deceleration in the case of the isochronous control, such a feeling of discomfort can be reduced in the case of the droop control.
Therefore, the engine rpm can be controlled to the target rpm with high accuracy by using the isochronous control with the electronic governor in some kinds of real machines or vehicles having the control device. However, rotation control (virtual droop control) similar to the droop control with the conventional mechanical governor characteristics is demanded for the general-purpose engine.
Conventionally, the general-purpose engine is generally used for vessels, farm vehicles, and construction-work vehicles. Therefore, compression-ignition engines (diesel engines) are mainly used as the general-purpose engine. In recent years, however, in view of environmental protection such as gas emission performance, a reduction in cost, and a reduction in size as well as in weight, a spark-ignition engine (gasoline/LPG) has been used. Therefore, when the spark-ignition engine (gasoline/LPG) is used as the general-purpose engine, output characteristics close to those of the conventional diesel engine and an operational feeling without a feeling of discomfort are demanded.
Conventionally, the mechanical governor is mainly used in the spark-ignition engine (gasoline/LPG) in which the rpm control is performed. In that case, an rpm of a throttle valve is controlled to a target rpm by an external mechanical governor mechanism. Therefore, when steady stability is considered as an important point, responsiveness is degraded. On the other hand, when the responsiveness is considered as an important point, the steady stability is lowered. In the worst case, hunting occurs in the rpm. Therefore, in terms of control accuracy for the target rpm, the diesel engine is better than the spark-ignition engine.